39° POPULAR SCIENCE MONTHLY 



amino-acids were formed in ordinary intestinal digestion. But such 

 data twenty-five years ago, and indeed up to very recent times, failed 

 to attract much attention or were misinterpreted. Physiologists ha- 

 stened to formulate a theory which would harmonize with existing views, 

 and so arose the theory of " luxus consumption," in which it was held 

 that when an excess of protein food was taken, far larger than the 

 demands of the body called for, the organism was able to protect itself 

 by virtue of this power possessed by trypsin of breaking down protein 

 matter into simple decomposition products easily got rid of with 

 less strain upon liver, kidneys and other organs and tissues. 



Many of you doubtless remember the experiments of Schmidt- 

 Mulheim and of Fano, who attempted to determine the amounts of 

 proteoses and peptones present in the blood of dogs after a hearty meal 

 of protein food ; and how the negative results they obtained led finally 

 to experiments on the injection of these substances directly into the 

 blood, in which it was found that marked physiological action followed. 

 In other words, proteoses and peptones are not normal constituents of 

 the blood, even though there be a large amount of them in the intestine. 

 They are plainly not absorbed as such, and this fact led to the theory — 

 apparently supported by experiment — that proteoses and peptones in the 

 very act of absorption, in their passage through the epithelial cells of the 

 intestinal wall, are transformed into the proteins of the blood. This 

 made a convenient way of explainirjg the facts, and one could well 

 imagine that the system took this method of reinforcing the proteins 

 of blood, lymph and tissue. Data, however, have been slowly accumu- 

 lating which do not admit of such easy interpretation. Physiological 

 chemists interested in enzyme action and equally interested in the 

 chemical constitution of protein matter have been gradually collecting 

 evidence of much significance. A row of diamino-acids, arginine, 

 lysine and histidine, together with alanine, proline, cystine, trypto- 

 phane, etc., have been discovered as hydrolytic decomposition products 

 of proteins, both by the action of pancreatic juice and by boiling dilute 

 acids, in addition to the earlier known leucine, tyrosine, glycocoll, 

 aspartic and glutaminic acids, etc. Further, it has been shown that 

 pancreatic juice in artificial digestion experiments, if sufficient time 

 be allowed, is able to bring about a complete breaking down of the 

 protein molecule into these relatively simple amino-acids, so that the 

 biuret reaction, for example, entirely disappears. For a time, this 

 extremely significant fact was not accredited with much importance 

 physiologically; it was interesting; it testified to the general lability 

 of the protein molecule and it threw light on the nature of the building 

 stones which make up the protein complex. Then came, as many of 

 you know, the comparatively recent discovery by Cohnheim of the 

 enzyme erepsin in the duodenal mucous membrane; an enzyme which 



